tty: fix "IRQ45: nobody cared"

Unthrottling the TTY during close ends up enabling interrupts
on a device not on the active list, which will never have the
interrupts cleared.  Doctor, it hurts when I do this.

>>> On 6/2/2011 at 01:56 AM, in message <20110601145608.3e586e16@bob.linux.org.uk>, Alan Cox <alan@linux.intel.com> wrote:
> On Wed, 01 Jun 2011 10:34:07 +1200
> "andrew mcgregor" <andrew.mcgregor@alliedtelesis.co.nz> wrote:
> > The LKML message
> > http://kerneltrap.org/mailarchive/linux-kernel/2010/2/25/4541847

 from
> > February doesn't seem to have been resolved since.  We struck the
> > issue, and the patch below (against 2.6.32) fixes it.  Should I
> > supply a patch against 3.0.0rc?
>
> I think that would be sensible. I don't actually see how you hit it as
> the IRQ ought to be masked by then but it's certainly wrong for n_tty
> to be calling into check_unthrottle at that point.
>
> So yes please send a patch with a suitable Signed-off-by: line to
> linux-serial and cc GregKH <greg@kroah.com> as well.
>
> Alan

Signed-off-by: Andrew McGregor <andrew.mcgregor@alliedtelesis.co.nz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>